1. Field of the Invention
The present invention relates generally to sense amplifiers and particularly to an MOS (metal oxide semiconductor) sense amplifier having unique precharging and reference voltage circuitry.
2. Description of the Prior Art
Sense amplifiers are commonly used where there is a need to sense a relatively small voltage difference between two voltage sources and amplify it to the level required by the system. A typical sense amp will contain one or more coupled, differential amplifier stages. The first stage having a transistor connected to a reference voltage and a transistor connected to the voltage to be compared. These two transistors are commonly termed the differential pair. Hysteresis, or variation in the reference voltage, is commonly controlled in the prior art by varying the voltage applied to a biasing transistor connected to the sources of the differential pair.
This method of achieving the hysteresis effect has certain limitations in that it is difficult to vary the reference voltage by only a few millivolts. In addition, due to processing variations in the manufacturing of metal oxide semiconductor circuitry, the variations in the hysteresis points may be significant. Also, the calculations related to defining a new hysteresis point are generally cumbersome and lengthy.
Another problem in the prior art is the amount of voltage required to reach the threshold of the comparison transistor. In a standard MOS differential amplifier this limitation generally means that signals below one and a half volts cannot be amplified.
Related to the sensing of key positions in keyboards used with video display terminals, two prior art techniques are known. One involves using make-or-break contacts for each key position. This method has several problems, such as susceptibility to contamination due to dust, moisture or other environmental factors and, over a period of time, degradation or contamination of the contacts due to electrical arcing at control closure. Also, a closed contact, undesirably, draws a large amount of power. The other method of key sensing involves using a sense amplifier constructed of discrete components. This method is also undesirable because of the labor costs involved in manufacture, the amount of power consumed in operation, and the space occupied by the circuitry. In addition, a discrete sense amp may drift out of specification over time.
The present invention relates to a novel sense amp circuit for resolving the above prior art problems.